In recent years, in vehicles equipped with electric motors as a motive power source such as hybrid vehicles and electric vehicles, the capacity of a power storage unit has been being increased to improve acceleration performance and running performance such as a maximum traveling distance. As means for increasing the capacity of a power storage unit, a configuration having a plurality of power storage devices has been proposed.
For example, Japanese Patent Laying-Open No. 2003-209969 discloses a power supply control system including a plurality of power stages. The power supply control system includes a plurality of power stages connected in parallel with one another and supplying direct current (DC) power to at least one inverter. Each power stage includes a battery and a boost/buck DC-DC converter.
In the power supply control system, the plurality of power stages are controlled to maintain an output voltage to the inverter by uniformly charging/discharging the batteries respectively included in the plurality of power stages.
Typically, in a boost/buck DC-DC converter as disclosed in Japanese Patent Laying-Open No. 2003-209969 described above, a dead time allowing for a switch turn-off delay time is provided to prevent a short circuit caused by simultaneous turning-on of a boost switch and a buck switch. When the boost/buck DC-DC converter has a low boost ratio, the output voltage vibrates under the influence of the dead time.
Accordingly, it is conceivable to always keep the buck switch ON when the boost ratio is low. However, if the buck switch is always kept ON in a configuration including a plurality of power stages as in the power supply control system disclosed in the publication described above, a short circuit occurs between the batteries, and an uncontrollable current flows between the batteries.